Optical scanning device having a temperature compensation unit

ABSTRACT

In an optical scanning device and image forming apparatus according to the present invention, a light source emits a light beam, and a scanning optical unit deflects the light beam from the light source and focuses the deflected light beam to form a light spot on a scanned surface, the scanned surface being scanned by the light beam from the scanning optical unit. A temperature detection unit detects a temperature of the scanning optical unit and its neighboring locations. A temperature compensation unit adjusts a focal-point position of the light beam on the scanned surface in accordance with a change in the temperature detected by the temperature detection unit, the temperature compensation unit adjusting the focal-point position of the light beam by directly varying a focusing effect of a corrector lens on the light beam from the light source by a controlled amount of movement of the corrector lens along its optical axis that corresponds to the temperature change.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of, and claims priority to,Ser. No. 09/716,949 filed Nov. 22, 2000, now U.S. Pat. No. 6,785,028,and claims priority to JP 11-333510 filed Nov. 24, 1999 and JP2000-023930 filed Feb. 1, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical scanning device in which ascanning optical unit that emits a light beam and focuses it on ascanned surface is provided with a temperature-compensation unit thateliminates a scanned-surface focal-point deviation of the light beamfocused by the scanning optical unit, due to a temperature change of thescanning optical unit and its neighboring locations. Further, thepresent invention relates to an image forming apparatus in which theoptical scanning device is provided with the temperature-compensationunit.

2. Description of the Related Art

In an image forming apparatus, such as laser printer, digital copier orlaser facsimile, a laser scanning device emits a laser beam inaccordance with an image signal, and focuses the laser beam onto ascanned surface of a photosensitive medium. In the laser scanningdevice, a collimator lens converts the laser beam, emitted by the laserlight source, into a collimated laser beam, and a rotary polygonalmirror, which is rotated at a high speed, deflects the collimated laserbeam to the photosensitive medium. The deflected beam is passed throughan fθ lens, and the fθ lens focuses the deflected beam to form a lightspot on the photosensitive medium surface. In the above laser scanningdevice, the collimator lens, the rotary polygonal mirror, and the fθlens constitute the scanning optical unit.

In an image forming apparatus including the laser scanning device, thephotosensitive medium surface is scanned in a main scanning direction bythe laser beam from the laser scanning device. In a synchronous mannerwith the time the main scanning is performed, the photosensitive mediumis rotated around its rotation axis, and the photosensitive mediumsurface is scanned in a sub-scanning direction by the laser beam fromthe laser scanning device. Hence, in the image forming apparatus, anelectrostatic image is formed on the photosensitive medium surface byusing the laser scanning device.

When the optical scanning device, like the above laser scanning device,is actually operated over an extended period of time, it is inevitablethat a scanned-surface focal-point position of the light beam focused bythe scanning optical unit deviates from a design-value position due toenvironmental changes, in particular, due to temperature changes of thescanning optical unit and its neighboring locations. In a certain case,the temperature changes cause the thermal deformation of the elements ofthe optical scanning device, and the light spot, which is formed on thescanned surface by the scanning optical unit, is larger than a requireddiameter. The contrast of the resulting image on the scanned surfacewill be lowered due to the focal-point deviation of the laser beam, andthis will degrade the quality of the resulting image that is formed bythe image forming apparatus using the optical scanning device.

As disclosed in Japanese Patent Publication No. 2692944, a scanningoptical device that is provided with a temperature compensation unit foreliminating a scanned-surface focal-point deviation of the light beamdue to a temperature change of the scanning optical device is known. Inthe temperature compensation unit of the above document, afocusing-condition detection means detects a focusing condition of thelaser beam on the scanned surface and outputs a detection signalindicative of the focusing condition. A corrector lens is movablyprovided in the collimator lens of the scanning optical device. When atemperature detection means detects a temperature change of the scanningoptical device, movement of the corrector lens along the optical axisrelative to the scanned surface is controlled by a feedback loop basedon the focusing-condition detection signal, until the focusing-conditiondetection means detects a desired focusing condition of the laser beamon the scanned surface in which the focal-point deviation of the lightbeam is cancelled. Alternatively, the temperature compensation unit ofthe above document may be achieved by either the movement of thecollimator lens along the optical axis or the movement of the laserlight source.

Further, as disclosed in Japanese Laid-Open Patent Application No.4-107581, a scanning optical device that is provided with a temperaturecompensation unit is known. In the conventional temperature compensationunit of the above document, a focusing-condition detection means detectsa focusing condition of the laser beam on the scanned surface of aphotosensitive medium by outputting a detection signal indicating thefocusing condition. A temperature control means, including the Peltierelement, is provided on the laser light source. An automatic focusingdevice including a corrector lens movably provided therein is operatedin response to a control signal output by the temperature control means,and the automatic focusing operation of the automatic focusing device iscontrolled in response to the detection signal output by thefocusing-condition detection means, such that the corrector lens ismoved along the optical axis by the automatic focusing device and thefocusing-condition detection means detects a desired focusing conditionof the laser beam on the scanned surface.

However, in the conventional scanning optical devices of the abovedocuments, when a temperature change of the scanning optical device isdetected, the automatic focusing operation must be performed to attainthe desired focusing condition of the laser beam on the scanned surface.The configuration of the conventional scanning optical devices and theautomatic focusing operation thereof are complicated, and much time istaken to reach the desired focusing condition of the laser beam by theautomatic focusing operation. Further, the movement of the correctorlens is directed to elimination of only one of a main-scanning-directionfocal-point deviation and a sub-scanning-direction focal-point deviationcaused by the temperature change. If a focal-point position of the laserbeam on the scanned surface that is optimum with respect to themain-scanning direction can be achieved, the resulting focal-pointposition of the laser beam on the scanned surface is not necessarilysuitable with respect to the sub-scanning direction.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean improved optical scanning device in which the above-describedproblems are eliminated.

Another object of the present invention is to provide an opticalscanning device which includes a simple, low-cost temperaturecompensation unit that can quickly achieve the optimum focal-pointposition of the light beam on the scanned surface by eliminating thefocal-point deviation due to temperature changes without performing theautomatic focusing operation.

Another object of the present invention is to provide an image formingapparatus in which an optical scanning device is provided with a simple,low-cost temperature compensation unit that can quickly achieve theoptimum focal-point position of the light beam on the scanned surface byeliminating the deviation due to temperature changes without performingthe automatic focusing operation.

The above-mentioned objects of the present invention are achieved by anoptical scanning device including: a light source which emits a lightbeam; a scanning optical unit which deflects the light beam from thelight source and focuses the deflected light beam to form a light spoton a scanned surface, the scanned surface being scanned by the lightbeam from the scanning optical unit; a temperature detection unit whichdetects a temperature of the scanning optical unit and its neighboringlocations; and a temperature compensation unit which adjusts afocal-point position of the light beam on the scanned surface inaccordance with a change in the temperature detected by the temperaturedetection unit, the temperature compensation unit adjusting thefocal-point position of the light beam by directly varying a focusingeffect of a corrector lens on the light beam from the light source by acontrolled amount of movement of the corrector lens along its opticalaxis that corresponds to the temperature change.

The above-mentioned objects of the present invention are achieved by anoptical scanning device which includes: a light source unit which has aplurality of light sources emitting multiple light beams; a scanningoptical unit which deflects the multiple light beams from the lightsources at a single location and focuses the deflected light beam toform a light spot on a scanned surface of a photosensitive medium, thescanned surface being scanned by the light beam from the scanningoptical unit; a temperature detection unit which detects a temperatureof the scanning optical unit and its neighboring locations; and atemperature compensation unit which adjusts each of amain-scanning-direction focal-point position, a sub-scanning-directionfocal-point position and a sub-scanning-direction beam pitch related tothe light beam on the scanned surface in accordance with a change in thetemperature detected by the temperature detection unit.

The above-mentioned objects of the present invention are achieved by animage forming apparatus in which an optical scanning device is provided,the optical scanning device including: a light source which emits alight beam; a scanning optical unit which deflects the light beam fromthe light source and focuses the deflected light beam to form a lightspot on a scanned surface, the scanned surface being scanned by thelight beam from the scanning optical unit; a temperature detection unitwhich detects a temperature of the scanning optical unit and itsneighboring locations; and a temperature compensation unit which adjustsa focal-point position of the light beam on the scanned surface inaccordance with a change in the temperature detected by the temperaturedetection unit, the temperature compensation unit adjusting thefocal-point position of the light beam by directly varying a focusingeffect of a corrector lens on the light beam from the light source by acontrolled amount of movement of the corrector lens along its opticalaxis that corresponds to the temperature change.

The above-mentioned objects of the present invention are achieved by animage forming apparatus in which an optical scanning device is provided,the optical scanning device including: a light source unit which has aplurality of light sources emitting multiple light beams; a scanningoptical unit which deflects the multiple light beams from the lightsources at a single location and focuses the deflected light beam toform a light spot on a scanned surface of a photosensitive medium, thescanned surface being scanned by the light beam from the scanningoptical unit; a temperature detection unit which detects a temperatureof the scanning optical unit and its neighboring locations; and atemperature compensation unit which adjusts each of amain-scanning-direction focal-point position, a sub-scanning-directionfocal-point position and a sub-scanning-direction beam pitch related tothe light beam on the scanned surface in accordance with a change in thetemperature detected by the temperature detection unit.

In the optical scanning device and the image forming apparatus accordingto the present invention, the temperature compensation unit adjusts thescanned-surface focal-point position of the light beam by directlyvarying the focusing effect of the corrector lens on the light beam fromthe light source by a controlled amount of movement of the correctorlens along its optical axis that corresponds to the temperature change.The temperature compensation unit of the present invention can beconstructed in a simple, inexpensive configuration. The temperaturecompensation unit of the present invention is effective in quicklyachieving the optimum focal-point position of the light beam on thescanned surface by eliminating the focal-point deviation due to atemperature change of the scanning optical unit. It is not necessary forthe optical scanning device of the present invention to perform theautomatic focusing operation when the temperature of the scanningoptical unit changes, as in the conventional optical scanning devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

FIG. 1 is a schematic diagram showing a first preferred embodiment ofthe optical scanning device of the present invention.

FIG. 2 is a diagram for explaining focal-point deviation characteristicsof the optical scanning device of the present embodiment that aredependent on a temperature change.

FIG. 3 is a schematic diagram showing a second preferred embodiment ofthe optical scanning device of the present invention.

FIG. 4 is a diagram for explaining focal-point deviation characteristicsof the optical scanning device of the present embodiment that aredependent on a temperature change.

FIG. 5 is a diagram for explaining corrector lens movementcharacteristics of the optical scanning device of the present embodimentthat are dependent on a temperature change.

FIG. 6 is a schematic diagram showing a third preferred embodiment ofthe optical scanning device of the present invention.

FIG. 7 is a diagram for explaining focal-point deviation characteristicsof the optical scanning device of the present embodiment that aredependent on a temperature change.

FIG. 8 is a schematic diagram showing a fourth preferred embodiment ofthe optical scanning device of the present invention.

FIG. 9 is a diagram for explaining a relationship betweenmain-scanning-direction focal-point deviation and laser-diode unitmovement in the present embodiment.

FIG. 10 is a diagram for explaining a relationship betweensub-scanning-direction focal-point deviation and laser-diode unitmovement in the present embodiment.

FIG. 11 is a diagram for explaining a relationship betweensub-scanning-direction beam-pitch deviation and laser-diode unitmovement in the present embodiment.

FIG. 12 is a diagram for explaining a relationship betweenmain-scanning-direction focal-point deviation and first corrector lensmovement in the present embodiment.

FIG. 13 is a diagram for explaining a relationship betweensub-scanning-direction focal-point deviation and first corrector lensmovement in the present embodiment.

FIG. 14 is a diagram for explaining a relationship betweensub-scanning-direction beam-pitch deviation and first corrector lensmovement in the present embodiment.

FIG. 15 is a diagram for explaining a relationship betweenmain-scanning-direction focal-point deviation and second corrector lensmovement in the present embodiment.

FIG. 16 is a diagram for explaining a relationship betweensub-scanning-direction focal-point deviation and second corrector lensmovement in the optical scanning device of the present embodiment.

FIG. 17 is a diagram for explaining a relationship betweensub-scanning-direction beam-pitch deviation and second corrector lensmovement in the optical scanning device of the present embodiment.

FIG. 18 is a diagram for explaining a relationship betweenmain-scanning-direction focal-point deviation of the optical scanningdevice of the present embodiment and temperature change.

FIG. 19 is a diagram for explaining a relationship betweensub-scanning-direction focal-point deviation of the optical scanningdevice of the present embodiment and temperature change.

FIG. 20 is a diagram for explaining a relationship betweensub-scanning-direction beam-pitch deviation of the optical scanningdevice and temperature change.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A description will now be given of preferred embodiments of the opticalscanning device and the image forming apparatus of the present inventionwith reference to the accompanying drawings.

FIG. 1 shows a first preferred embodiment of the optical scanning deviceof the present invention. The optical scanning device of the presentembodiment is provided for use in an image forming apparatus, such as alaser printer, a digital copier or a laser facsimile. In the imageforming apparatus, an image is formed on a scanned surface of aphotosensitive medium when the photosensitive medium surface is scannedin the main scanning direction and the sub-scanning direction by thelight beam focused by the optical scanning device.

As shown in FIG. 1, the optical scanning device of the presentembodiment generally comprises a light source 1, a coupling lens 2, acorrector lens 3, a deflector 4, a focusing lens system 5, aphotosensitive medium 6, a sync signal detector 7, a temperature sensor8, a corrector lens actuator 9, and a temperature compensation controlunit 10. In the optical scanning device of FIG. 1, the coupling lens 2,the deflector 4 and the focusing lens system 5 constitute the scanningoptical unit in the claims, while the corrector lens 3, the correctorlens actuator 9 and the temperature compensation control unit 10constitute the temperature compensation unit in the claims.

In the optical scanning device of FIG. 1, the light source 1 emits alight beam, such as a laser beam, in accordance with an image signal.The light source 1 in the present embodiment is a semiconductor laser(or a laser diode) that emits a laser beam. The coupling lens 2 couplesthe light beam emitted by the light source 1, and the coupled light beamis suitably introduced to the corrector lens 3.

The corrector lens 3 provides a refraction power to the coupled lightbeam from the coupling lens 2, with respect to both the main scanningdirection and the sub-scanning direction. In the present embodiment, thefocusing effect of the corrector lens 3 on the light beam from the lightsource 1 is varied by the corrector lens actuator 9 by a controlledamount of movement of the corrector lens 3 along its optical axis thatcorresponds to a detected temperature change. The movement of thecorrector lens 3 by the corrector lens actuator 9 is controlled by thetemperature compensation control unit 10 in accordance with thetemperature change. Accordingly, the focal-point position of the lightbeam on the scanned surface of the photosensitive medium 6 is adjustedby the temperature compensation control unit 10 in accordance with thetemperature change, so as to eliminate the focal-point deviation due tothe temperature change.

The deflector 4 in the present embodiment is a rotary polygonal mirrorhaving reflection surfaces on the six peripheral sides. One of thereflection surfaces of the deflector 4 deflects the beam from thecorrector lens 3 while the deflector 4 is rotated around its rotationaxis (which is perpendicular to the plane of illustration of FIG. 1).The deflected beam from the deflector 4 scans the scanned surface of thephotosensitive medium 6 in the main scanning direction (which isparallel to the axial direction of the photosensitive medium 6). In asynchronous manner with the time the main scanning is performed (orevery time the light beam from the deflector 4 is incident to the syncsignal detector 7), the photosensitive medium 6 is rotated around itsrotation axis by a given rotational angle. Hence, the photosensitivemedium surface is scanned in the main scanning direction and in thesub-scanning direction by the light beam focused by the optical scanningdevice.

The focusing lens system 5 includes an fθ lens that converts thedeflected light beam from the deflector 4 into a convergent light beamso that the convergent light beam from the focusing lens system 5 formsa light spot on the scanned surface of the photosensitive medium 6. Whenthe deflector 4 is rotated, the light beam passed through the focusinglens system 5 scans the photosensitive medium surface in the mainscanning direction.

The photosensitive medium 6 in the present embodiment is aphotoconductor drum that serves as an image support for an image on thescanned surface. An electrostatic latent image is formed on the scannedsurface of the photosensitive medium 6 when the photosensitive mediumsurface is exposed to a pattern of the light beam focused by the opticalscanning device.

The sync signal detector 7 outputs a main-scanning sync signal everytime the light beam from the deflector 4 is incident to the sync signaldetector 7 during the rotation of the deflector 4. The photosensitivemedium 6 is rotated around the rotation axis by the given rotationalangle in synchronism with the main-scanning sync signal output by thesync signal detector 7.

The temperature sensor 8 is provided in the vicinity of the sync signaldetector 7. The temperature sensor 8 outputs a signal indicative of atemperature of the scanning optical unit and its neighboring locations,to the temperature-compensation control unit 10. When the temperaturesensor 8 senses a first temperature of the scanning optical unit, theoutput signal of the temperature sensor 8 represents a first voltage orresistance of a thermoelectric element contained in the temperaturesensor 8. When the temperature of the scanning optical unit changes to asecond temperature and it is sensed by the temperature sensor 8, theoutput signal of the temperature sensor 8 represents a second voltage orresistance of the thermoelectric element. Hence, the temperature changeis detected by the temperature-compensation control unit 10 from achange of the voltage or resistance of the thermoelectric elementindicated by the output signal of the temperature sensor 8.

The temperature compensation control unit 10 outputs a control signal tothe corrector lens actuator 9 in accordance with the signal suppliedfrom the temperature sensor 8, the control signal indicating acontrolled amount of movement of the corrector lens 3 required to cancelthe focal-point deviation of the light beam due to the temperaturechange. The corrector lens actuator 9 moves the corrector lens 3 alongthe optical axis in accordance with the control signal supplied from thetemperature compensation control unit 10. Therefore, the temperaturecompensation control unit 10 controls the movement of the corrector lens3 by means of the corrector lens actuator 9 in accordance with thetemperature change detected by the temperature sensor 8, and thefocal-point position of the light spot on the scanned surface of thephotosensitive medium 6 is adjusted by the temperature compensationcontrol unit 10 so as to eliminate the focal-point deviation caused bythe temperature change.

In the present embodiment, the temperature-compensation control unit 10includes a memory that stores a table defining the relationship betweenthe temperature change and a corresponding focal-point deviation of thelight beam on the scanned surface. A simulation test of the opticalscanning device of the present embodiment is performed, in advance, andindividual scanned-surface focal-point deviations of the laser beamfocused by the scanning optical unit are measured at differenttemperatures of the scanning optical unit. The table defining the aboverelationship is created based on the results of the measurement of thesimulation test, and stored in the memory of thetemperature-compensation control unit 10. During the measurement of thesimulation test, the focusing effect of the corrector lens 3 on thelight beam from the light source 1 is fixed to a reference level and notvaried.

FIG. 2 shows focal-point deviation characteristics of the opticalscanning device of the present embodiment that are dependent on atemperature change of the scanning optical unit.

As described above, in the present embodiment, the simulation test isperformed, in advance, such that individual scanned-surface focal-pointdeviations of the laser beam focused by the scanning optical unit aremeasured at different temperatures of the scanning optical unit. Thetable defining the above relationship is created based on the results ofthe measurement of the simulation test, and the table, such as shown inFIG. 2, is stored in the memory of the temperature-compensation controlunit 10.

In the optical scanning device of FIG. 1, when a temperature change ofthe scanning optical unit is detected by the temperature sensor 8, thetemperature compensation control unit 10 adjusts the scanned-surfacefocal-point position of the light beam based on the focal-pointdeviation read from the table of the above memory in response to thetemperature change. Specifically, the temperature compensation controlunit 10 directly controls the corrector lens actuator 9 in accordancewith the signal output from the temperature sensor 8, so that thefocusing effect of the corrector lens 3 on the light beam from the lightsource 1 is directly varied by a controlled amount of movement of thecorrector lens 3 along the optical axis that corresponds to thetemperature change.

It is not necessary for the optical scanning device of the presentembodiment to perform the automatic focusing operation when thetemperature of the scanning optical unit changes. In the presentembodiment, it is possible to provide one-to-one correspondence betweenthe temperature of the scanning optical unit and the correspondingamount of movement of the corrector lens 3. The movement of thecorrector lens 3 controlled by the temperature compensation control unit10 results in the elimination of the focal-point deviation correspondingto that read from the memory. In the optical scanning device of thepresent embodiment, the temperature sensor 8, the corrector lensactuator 9 and the temperature compensation control unit 10 can beconstructed in a simple, inexpensive configuration. The optical scanningdevice and the image forming apparatus of the present embodiment areeffective in quickly achieving the optimum focal-point position of thelight beam on the scanned surface of the photosensitive medium 6 when atemperature change of the scanning optical unit is detected.

In the above-described embodiment, the temperature sensor 8 is providedin the vicinity of the sync signal detector 7 as shown in FIG. 1.Alternatively, the temperature sensor 8 may be provided in the vicinityof the focusing lens system 5 or in the vicinity of the light source 1which is subjected to significant temperature changes. Further, in orderto increase the accuracy of temperature detection, a plurality oftemperature sensors may be provided at different internal locationswithin the optical scanning device which are subjected to significanttemperature changes.

Next, FIG. 3 shows a second preferred embodiment of the optical scanningdevice of the present invention.

As shown in FIG. 3, the optical scanning device of the presentembodiment generally comprises the light source 1, the coupling lens 2,a first corrector lens 11, a second corrector lens 12, the deflector 4,the focusing lens system 5, the photosensitive medium 6, the sync signaldetector 7, the temperature sensor 8, a first corrector lens actuator13, a second corrector lens actuator 14, and a temperature compensationcontrol unit 15. In FIG. 3, the elements which are essentially the sameas corresponding elements in FIG. 1 are designated by the same referencenumerals, and a description thereof will be omitted.

In the optical scanning device of FIG. 3, the coupling lens 2, thedeflector 4 and the focusing lens system 5 constitute the scanningoptical unit in the claims, while the first and second corrector lenses11 and 12, the first and second corrector lens actuators 13 and 14 andthe temperature compensation control unit 15 constitute the temperaturecompensation unit in the claims.

In the optical scanning device of FIG. 3, the coupling lens 2 couplesthe light beam emitted by the light source 1, and the coupled light beamis suitably introduced to the first corrector lens 11.

The first corrector lens 11 provides a refraction power to the coupledlight beam from the coupling lens 2, with respect to the main scanningdirection. The second corrector lens 12 provides a refraction power tothe coupled light beam from the coupling lens 2, with respect to thesub-scanning direction. In the present embodiment, the focusing effectsof the corrector lenses 11 and 12 on the light beam from the lightsource 1 with respect to the main scanning direction and thesub-scanning direction are individually varied by the first and secondcorrector lens actuators 13 and 14 by controlled amounts of movement ofthe corrector lenses 11 and 12 along the optical axis that correspond toa detected temperature change. The movements of the corrector lenses 11and 12 by the first and second corrector lens actuators 13 and 14 areindividually controlled by the temperature compensation control unit 15in accordance with the temperature change. Accordingly, themain-scanning-direction and sub-scanning-direction focal-point positionsof the light beam on the scanned surface of the photosensitive medium 6are adjusted by the temperature compensation control unit 15 inaccordance with the temperature change, so as to eliminate thefocal-point deviations due to the temperature change.

The temperature compensation control unit 15 outputs a first controlsignal and a second control signal to the first corrector lens actuator13 and the second corrector lens actuator 14, respectively, inaccordance with the signal supplied from the temperature sensor 8. Thefirst control signal output by the control unit 15 indicates acontrolled amount of movement of the first corrector lens 11 required tocancel the focal-point deviation of the light beam in the main scanningdirection due to the temperature change. The second control signaloutput by the control unit 15 indicates a controlled amount of movementof the second corrector lens 12 required to cancel the focal-pointdeviation of the light beam in the sub-scanning direction due to thetemperature change. The first corrector lens actuator 13 moves the firstcorrector lens 11 along the optical axis in accordance with the firstcontrol signal supplied from the temperature compensation control unit15. The second corrector lens actuator 14 moves the second correctorlens 12 along the optical axis in accordance with the second controlsignal supplied from the temperature compensation control unit 15. Thetemperature compensation control unit 15 individually controls themovement of the first corrector lens 11 and the movement of the secondcorrector lens 12 in accordance with the temperature change detected bythe temperature sensor 8. Therefore, the main-scanning-direction andsub-scanning-direction focal-point positions of the light beam on thescanned surface of the photosensitive medium 6 are adjusted by thetemperature compensation control unit 15 so as to eliminate thefocal-point deviations of the light beam with respect to the mainscanning direction and the sub-scanning direction.

In the present embodiment, the temperature-compensation control unit 15includes a memory that stores a table defining the relationship betweenthe temperature change and corresponding focal-point deviations (withrespect to the main scanning direction and the sub-scanning direction)of the light beam on the scanned surface. A simulation test of theoptical scanning device of the present embodiment is performed, inadvance, and individual scanned-surface focal-point deviations of thelaser beam focused by the scanning optical unit are measured atdifferent temperatures of the scanning optical unit. The table definingthe above relationship is created based on the results of themeasurement of the simulation test, and stored in the memory of thetemperature-compensation control unit 15. During the measurement of thesimulation test, the focusing effects of the corrector lenses 11 and 12on the light beam from the light source 1 are fixed to a reference leveland not varied.

FIG. 4 shows focal-point deviation characteristics of the opticalscanning device of the present embodiment that are dependent on atemperature change of the scanning optical unit.

FIG. 5 shows corrector lens movement characteristics of the opticalscanning device of the present embodiment that are dependent on atemperature change of the scanning optical unit.

As described above, in the present embodiment, the simulation test isperformed, in advance, such that individual scanned-surface focal-pointdeviations (with respect to the main scanning direction and thesub-scanning direction) of the laser beam focused by the scanningoptical unit are measured at different temperatures of the scanningoptical unit. The table defining the above relationship is created basedon the results of the measurement of the simulation test, and the table,such as shown in FIG. 4, is stored in the memory of thetemperature-compensation control unit 15.

Alternatively, the simulation test may be performed, in advance, suchthat individual controlled amounts of movement of each of the firstcorrector lens 11 and the second corrector lens 12 are calculated withrespect to the respective focal-point deviations measured at differenttemperatures of the scanning optical unit. In such alternativeembodiment, the table defining the above relationship is created basedon the results of the measurement of the simulation test, and the table,such as shown in FIG. 5, is stored in the memory of thetemperature-compensation control unit 15.

In the optical scanning device of FIG. 3, when a temperature change ofthe scanning optical unit is detected by the temperature sensor 8, thetemperature compensation control unit 15 adjusts themain-scanning-direction and sub-scanning-direction focal-point positionsof the light beam based on the focal-point deviations read from thetable of the memory (FIG. 4) in response to the temperature change.Specifically, the temperature compensation control unit 15 directlycontrols the corrector lens actuators 13 and 14 in accordance with thesignal output from the temperature sensor 8, so that the focusingeffects of the corrector lenses 11 and 12 on the light beam from thelight source 1 are directly varied by the controlled amounts of movementof the first and second corrector lenses 11 and 12 along the opticalaxis that correspond to the temperature change.

It is not necessary for the optical scanning device of the presentembodiment to perform the automatic focusing operation when thetemperature of the scanning optical unit changes. In the presentembodiment, it is possible to provide one-to-one correspondence betweenthe temperature of the scanning optical unit and the correspondingamount of movement of each of the corrector lenses 11 and 12. Themovements of the corrector lenses 11 and 12 controlled by thetemperature compensation control unit 15 result in the elimination ofthe focal-point deviations corresponding to those read from the memory.In the optical scanning device of the present embodiment, thetemperature sensor 8, the corrector lens actuators 13 and 14 and thetemperature compensation control unit 15 can be constructed in a simple,inexpensive configuration. The optical scanning device and the imageforming apparatus of the present embodiment are effective in quicklyachieving the optimum focal-point position of the light beam on thescanned surface of the photosensitive medium 6 when a temperature changeof the scanning optical unit is detected.

In the above-described embodiment, the temperature sensor 8 is providedin the vicinity of the sync signal detector 7 as shown in FIG. 3.Alternatively, the temperature sensor 8 may be provided in the vicinityof the focusing lens system 5 or in the vicinity of the light source 1which is subjected to significant temperature changes. Further, in orderto increase the accuracy of temperature detection, a plurality oftemperature sensors may be provided at different internal locationswithin the optical scanning device which are subjected to significanttemperature changes.

Next, FIG. 6 shows a third preferred embodiment of the optical scanningdevice of the present invention.

As shown in FIG. 6, the optical scanning device of the presentembodiment generally comprises the light source 1, the coupling lens 2,a second corrector lens 12′, the deflector 4, the focusing lens system5, the photosensitive medium 6, the sync signal detector 7, thetemperature sensor 8, a second corrector lens actuator 14′, and atemperature compensation control unit 16. In the present embodiment, thefirst corrector lens 11 and the first corrector lens actuator 13 as inthe previous embodiment of FIG. 3 are omitted. In FIG. 6, the elementswhich are essentially the same as corresponding elements in FIG. 1 aredesignated by the same reference numerals, and a description thereofwill be omitted.

In the optical scanning device of FIG. 6, the coupling lens 2, thedeflector 4 and the focusing lens system 5 constitute the scanningoptical unit in the claims, while the second corrector lens 12′, thesecond corrector lens actuator 14′ and the temperature compensationcontrol unit 16 constitute the temperature compensation unit in theclaims.

In the optical scanning device of FIG. 6, a light source module in whichthe light source 1 and the coupling lens 2 are accommodated is formedwith a most suitable linear expansivity such that the focal-pointdeviation of the light beam on the scanned surface in the main scanningdirection for the temperature change is made almost negligible orvirtually equal to zero. Hence, the first corrector lens 11 and thefirst corrector lens actuator 13 as in the previous embodiment of FIG. 3are omitted in the present embodiment.

In the optical scanning device of FIG. 6, the coupling lens 2 couplesthe light beam emitted by the light source 1, and the coupled light beamis suitably introduced to the second corrector lens 12′. The secondcorrector lens 12′ provides a refraction power to the coupled light beamfrom the coupling lens 2, with respect to the sub-scanning direction. Inthe present embodiment, the focusing effect of the corrector lens 12′ onthe light beam from the light source 1 with respect to the sub-scanningdirection is varied by the second corrector lens actuator 14′ by acontrolled amount of movement of the corrector lens 12′ along theoptical axis that corresponds to a detected temperature change. Themovement of the corrector lens 12′ by the second corrector lens actuator14′ is controlled by the temperature compensation control unit 16 inaccordance with the temperature change. In the present embodiment, thesub-scanning-direction focal-point position of the light beam on thescanned surface of the photosensitive medium 6 is adjusted by thetemperature compensation control unit 16 in accordance with thetemperature change, so as to eliminate the focal-point deviation due tothe temperature change. In the present embodiment, the light sourcemodule in which the light source 1 and the coupling lens 2 areaccommodated is formed with the most suitable linear expansivity suchthat the focal-point deviation of the light beam on the scanned surfacein the main scanning direction for the temperature change is made almostnegligible.

The temperature compensation control unit 16 outputs a control signal tothe second corrector lens actuator 14′ in accordance with the signalsupplied from the temperature sensor 8. The control signal output by thecontrol unit 16 indicates a controlled amount of movement of the secondcorrector lens 12′ required to cancel the focal-point deviation of thelight beam in the sub-scanning direction due to the temperature change.The second corrector lens actuator 14′ moves the second corrector lens12′ along the optical axis in accordance with the control signalsupplied from the temperature compensation control unit 16. Thetemperature compensation control unit 16 controls the movement of thesecond corrector lens 12′ in accordance with the temperature changedetected by the temperature sensor 8. Therefore, thesub-scanning-direction focal-point position of the light beam on thescanned surface of the photosensitive medium 6 is adjusted by thetemperature compensation control unit 16 so as to eliminate thefocal-point deviation of the light beam with respect to the sub-scanningdirection due to the temperature change.

In the present embodiment, the temperature-compensation control unit 16includes a memory that stores a table defining the relationship betweenthe temperature change and a corresponding focal-point deviation (withrespect to the sub-scanning direction) of the light beam on the scannedsurface. A simulation test of the optical scanning device of the presentembodiment is performed, in advance, and individual scanned-surfacefocal-point deviations of the laser beam focused by the scanning opticalunit are measured at different temperatures of the scanning opticalunit. The table defining the above relationship is created based on theresults of the measurement of the simulation test, and stored in thememory of the temperature-compensation control unit 16. During themeasurement of the simulation test, the focusing effect of the correctorlens 12 on the light beam from the light source 1 is fixed to areference level and not varied.

FIG. 7 shows focal-point deviation characteristics of the opticalscanning device of the present embodiment that are dependent on atemperature change.

In the optical scanning device of FIG. 6, when a temperature change ofthe scanning optical unit is detected by the temperature sensor 8, thetemperature compensation control unit 16 adjusts thesub-scanning-direction focal-point position of the light beam based onthe focal-point deviation read from the table of the memory (FIG. 7) inresponse to the temperature change. Specifically, the temperaturecompensation control unit 16 directly controls the corrector lensactuator 14′ in accordance with the signal output from the temperaturesensor 8, so that the focusing effect of the corrector lens 12′ on thelight beam from the light source 1 is directly varied by the controlledamount of movement of the second corrector lens 12′ along the opticalaxis that correspond to the temperature change. The adjustment of themain-scanning-direction focal-point position of the light beam inaccordance with the temperature change is not needed for the opticalscanning device of the present embodiment.

It is not necessary for the optical scanning device of the presentembodiment to perform the automatic focusing operation when thetemperature of the scanning optical unit changes. In the presentembodiment, it is possible to provide one-to-one correspondence betweenthe temperature of the scanning optical unit and the correspondingamount of movement of the corrector lens 12′. The movement of thecorrector lens 12′ controlled by the temperature compensation controlunit 16 results in the elimination of the focal-point deviationcorresponding to that read from the memory. In the optical scanningdevice of the present embodiment, the temperature sensor 8, thecorrector lens actuator 14′ and the temperature compensation controlunit 16 can be constructed in a simple, inexpensive configuration. Theoptical scanning device and the image forming apparatus of the presentembodiment are effective in quickly achieving the optimum focal-pointposition of the light beam on the scanned surface of the photosensitivemedium 6 when a temperature change of the scanning optical unit isdetected.

In the above-described embodiment, the temperature sensor 8 is providedin the vicinity of the sync signal detector 7 as shown in FIG. 3.Alternatively, the temperature sensor 8 may be provided in the vicinityof the focusing lens system 5 or in the vicinity of the light source 1which is subjected to significant temperature changes. Further, in orderto increase the accuracy of temperature detection, a plurality oftemperature sensors may be provided at different internal locationswithin the optical scanning device which are subjected to significanttemperature changes.

In the above-described embodiments of FIG. 1, FIG. 3 and FIG. 6, thetemperature sensor 8 is integrally formed on either an integratedcircuit board in which the sync signal detector 7 or the deflector 4 isprovided or an integrated circuit board in which the light source 1 (thelaser diode) is provided. It is not necessary to provide an additionalcircuit board for mounting the temperature sensor 8 only in order toconstruct the optical scanning device of the present invention. Thetemperature compensation unit of the present invention can beconstructed in a simple, inexpensive configuration.

In the above-described embodiment of FIG. 6, the first corrector lens 11and the first corrector lens actuator 13 as in the previous embodimentof FIG. 3 are omitted, and the light source module in which the lightsource 1 and the coupling lens 2 are accommodated is formed with a mostsuitable linear expansivity such that the focal-point deviation of thelight beam on the scanned surface in the main scanning direction for thetemperature change is made almost negligible. However, the presentinvention is not limited to this embodiment. Alternatively, the secondcorrector lens 12 and the second corrector lens actuator 14 as in theprevious embodiment of FIG. 3 may be omitted. In such alternativeembodiment, the light source module in which the light source 1 and thecoupling lens 2 are accommodated is formed with a most suitable linearexpansivity such that the focal-point deviation of the light beam on thescanned surface in the sub-scanning direction for the temperature changeis made almost negligible.

Next, FIG. 8 shows a fourth preferred embodiment of the optical scanningdevice of the present invention.

As shown in FIG. 8, the optical scanning device of the presentembodiment generally comprises a light source unit (also called thelaser diode unit) 21, a first corrector lens 22, a second corrector lens23, a deflector 24, a focusing lens system 25, a photosensitive medium26, a temperature sensor 27, a temperature measuring unit 28, adeviation calculating unit 29, a beam-diameter control unit 30, abeam-pitch control unit 31, a first actuator 32, a second actuator 33, athird actuator 34, a reflector mirror 35, and a dust-proof glass 36. Inthe optical scanning device of FIG. 8, the deflector 24, the focusinglens system 25 and the reflector mirror 35 constitute the scanningoptical unit in the claims, the temperature sensor 27 and thetemperature measuring unit 28 constitute the temperature detection unitin the claims, while the first corrector lens 22, the second correctorlens 23, the first through third actuators 32-34, the deviationcalculating unit 29, the beam-diameter control unit 30 and thebeam-pitch control unit 31 constitute the temperature compensation unitin the claims.

In the optical scanning device of FIG. 8, the light source unit (or thelaser diode unit) 21 includes a plurality of light sources (for example,laser diodes) which emit multiple light beams, such as multiple laserbeams, in accordance with an image signal. The multiple light beams fromthe light source unit 21 are suitably introduced to the first correctorlens 22. The first actuator 32 moves the light source unit 21 along itsoptical axis in accordance with a control signal supplied from thebeam-pitch control unit 31. The temperature compensation unit of thepresent embodiment controls the movement of the light source unit 21 bymeans of the first actuator 32 in accordance with a detected temperaturechange, and adjusts the beam pitch of the light beams on the scannedsurface of the photosensitive medium 26 in the sub-scanning direction soas to eliminate the beam-pitch deviation caused by the temperaturechange.

The first corrector lens 22 provides a refraction power to the multiplelight beams from the light source unit 21, with respect to the mainscanning direction. The second corrector lens 23 provides a refractionpower to the multiple light beams beam from the light source unit 21,with respect to the sub-scanning direction. The first and secondcorrector lenses 22 and 23 in the present embodiment are constituted bycylindrical lenses. The second actuator 33 moves the first correctorlens 22 along its optical axis in accordance with a control signalsupplied from the beam-diameter control unit 30. The third actuator 34moves the second corrector lens 23 along its optical axis in accordancewith a control signal supplied from the beam-diameter control unit 30.

In the present embodiment, the focusing effect of the first correctorlens 22 on the multiple light beams from the light source unit 21 withrespect to the main scanning direction is varied by the second actuator33 by a controlled amount of movement of the first corrector lens 22along the optical axis that corresponds to a temperature change. Thefocusing effect of the second corrector lens 23 on the multiple lightbeams from the light source unit 21 with respect to the sub-scanningdirection is varied by the third actuator 34 by a controlled amount ofmovement of the second corrector lens 23 along the optical axis thatcorresponds to a temperature change. The movement of the first correctorlens 22 by the second actuator 33 and the movement of the secondcorrector lens 23 by the third actuator 34 are controlled by thebeam-diameter control unit 30 in accordance with the temperature change.

Therefore, the temperature compensation unit of the present embodimentcontrols the movement of the first corrector lens 22 by means of thesecond actuator 33 in accordance with a detected temperature change, andadjusts the focal-point position of the light beam on the scannedsurface of the photosensitive medium 26 in the main scanning directionso as to eliminate the main-scanning-direction focal-point deviation dueto the temperature change. At the same time, the temperaturecompensation unit of the present embodiment controls the movement of thesecond corrector lens 23 by means of the third actuator 34 in accordancewith the detected temperature change, and adjusts the focal-pointposition of the light beam on the scanned surface of the photosensitivemedium 26 in the sub-scanning direction so as to eliminate thesub-scanning-direction focal-point deviation due to the temperaturechange.

Accordingly, the temperature compensation unit of the present embodimentadjusts each of the main-scanning-direction focal-point position, thesub-scanning-direction focal-point position and thesub-scanning-direction beam pitch related to the light beam on thescanned surface in accordance with a detected temperature change.

The deflector 24 in the present embodiment is a rotary polygonal mirrorhaving reflection surfaces on the six peripheral sides. One of thereflection surfaces of the deflector 24 deflects the multiple lightbeams from the light source unit 21 at a single location while thedeflector 24 is rotated around its rotation axis as indicated by thearrow in FIG. 8. The deflected beam from the deflector 24 scans thescanned surface of the photosensitive medium 26 in the main scanningdirection (which is parallel to the axial direction of thephotosensitive medium 26). In a synchronous manner with the time themain scanning is performed, the photosensitive medium 26 is rotatedaround its rotation axis by a given rotational angle. Hence, thephotosensitive medium surface is scanned in the main scanning directionand in the sub-scanning direction by the light beam focused by theoptical scanning device.

The focusing lens system 25 includes an fθ lens that converts thedeflected light beam from the deflector 24 into a convergent light beamso that the convergent light beam from the focusing lens system 25 formsa light spot on the scanned surface of the photosensitive medium 26. Thereflector mirror 35 reflects the convergent light beam, passed throughthe focusing lens system 25, to the scanned surface of thephotosensitive medium 26. The dust-proof glass 36 is placed between thereflector mirror 35 and the photosensitive medium 26 to protect thescanned surface against dust. The reflected light beam from thereflector mirror 35 is allowed to pass through the dust-proof glass 36.During the rotation of the deflector 24, the light beam passed throughthe focusing lens system 25 scans the photosensitive medium surface inthe main scanning direction.

In the optical scanning device of FIG. 8, the temperature sensor 27 isprovided in the vicinity of the photosensitive medium 26. Thetemperature sensor 27 outputs a signal indicative of a temperature ofthe scanning optical unit and its neighboring locations, to thetemperature measuring unit 28. When the temperature sensor 27 senses afirst temperature of the scanning optical unit, the output signal of thetemperature sensor 27 represents a first voltage or resistance of athermoelectric element contained in the temperature sensor 27. When thetemperature of the scanning optical unit changes to a second temperatureand it is sensed by the temperature sensor 27, the output signal of thetemperature sensor 27 represents a second voltage or resistance of thethermoelectric element. Hence, a change of the voltage or resistance ofthe thermoelectric element indicated by the output signal of thetemperature sensor 27 is detected by the temperature measuring unit 28as a change of the temperature sensed by the temperature sensor 27.

The temperature measuring unit 28 outputs a signal indicative of thetemperature change to the deviation calculating unit 29. Based on thetemperature change indicated by the output signal of the temperaturemeasuring unit 28, the deviation calculating unit 29 respectivelycalculates a first amount of movement of the first corrector lens 22required to cancel the main-scanning-direction focal-point deviation ofthe light beam (on the scanned surface) due to the temperature change, asecond amount of movement of the second corrector lens 23 required tocancel the sub-scanning-direction focal-point deviation of the lightbeam (on the scanned surface) due to the temperature change, and a thirdamount of movement of the light source unit 21 required to cancel thesub-scanning-direction beam-pitch deviation of the light beam (on thescanned surface) due to the temperature change. The focal-pointdeviation calculating unit 28 supplies the calculated first and secondamounts of the corrector lens movement to the beam-diameter control unit30, and supplies the calculated third amount of the light source unitmovement to the beam-pitch control unit 31.

As the third amount of the light source unit movement from the deviationcalculating unit 29 is received at the beam-pitch control unit 31, thebeam-pitch control unit 31 controls the movement of the light sourceunit 21 by means of the first actuator 32 in accordance with thedetected temperature change, and adjusts the beam pitch of the lightbeam on the scanned surface of the photosensitive medium 26 in thesub-scanning direction so as to eliminate the beam-pitch deviationcaused by the temperature change.

As the first and second amounts of the corrector lens movement from thedeviation calculating unit 29 are received at the beam-diameter controlunit 30, the beam-diameter control unit 30 respectively controls themovement of the first corrector lens 22 and the movement of the secondcorrector lens 23 by means of the second actuator 33 and the thirdactuator 34 in accordance with the detected temperature change, andrespectively adjusts the main-scanning-direction focal-point positionand the sub-scanning-direction focal-point position related to the lightbeam on the scanned surface of the photosensitive medium 26 so as toeliminate the main-scanning-direction and sub-scanning-directionfocal-point deviations caused by the temperature change.

Next, a description will be provided of the calculation executed by thedeviation calculating unit 29 in the present embodiment.

FIG. 9 shows a relationship between main-scanning-direction focal-pointdeviation and laser-diode unit movement in the present embodiment. InFIG. 9, “a11” indicates a gradient of the main-scanning-directionfocal-point deviation to the laser-diode unit movement. FIG. 10 shows arelationship between sub-scanning-direction focal-point deviation andlaser-diode unit movement in the present embodiment. In FIG. 10, “a21”indicates a gradient of the sub-scanning-direction focal-point deviationto the laser-diode unit movement. FIG. 11 shows a relationship betweensub-scanning-direction beam-pitch deviation and laser-diode unitmovement in the present embodiment. In FIG. 11, “a31” indicates agradient of the sub-scanning-direction beam-pitch deviation to thelaser-diode unit movement.

In the present embodiment, the deviation calculating unit 29 includes amemory that stores the gradient “a11” defining the relationship betweenmain-scanning-direction focal-point deviation and laser-diode unitmovement, the gradient “a21” defining the relationship betweensub-scanning-direction focal-point deviation and laser-diode unitmovement, and the gradient “a31” defining the relationship betweensub-scanning-direction beam-pitch deviation and laser-diode unitmovement, respectively. A simulation test (or actual measurement) of theoptical scanning device of the present embodiment is performed, inadvance, and individual scanned-surface focal-point (or beam-pitch)deviations of the laser beam focused by the scanning optical unit aremeasured at different amounts of movement of the laser-diode unit (orthe light source unit 21). The gradients “a11”, “a21” and “a31” definingthe respective relationships are created based on the results of themeasurement of the simulation test, and stored in the memory of thedeviation calculating unit 29.

Further, FIG. 12 shows a relationship between main-scanning-directionfocal-point deviation and first corrector lens movement in the presentembodiment. In FIG. 12, “a12” indicates a gradient of themain-scanning-direction focal-point deviation to the first correctorlens movement. FIG. 13 shows a relationship betweensub-scanning-direction focal-point deviation and first corrector lensmovement in the present embodiment. In FIG. 13, “a22” indicates agradient of the sub-scanning-direction focal-point deviation to thefirst corrector lens movement. FIG. 14 shows a relationship betweensub-scanning-direction beam-pitch deviation and first corrector lensmovement in the present embodiment. In FIG. 14, “a32” indicates agradient of the sub-scanning-direction beam-pitch deviation to the firstcorrector lens movement.

In the present embodiment, the memory of the deviation calculating unit29 further stores the gradient “a12” defining the relationship betweenmain-scanning-direction focal-point deviation and first corrector lensmovement, the gradient “a22” defining the relationship betweensub-scanning-direction focal-point deviation and first corrector lensmovement, and the gradient “a32” defining the relationship betweensub-scanning-direction beam-pitch deviation and first correctormovement, respectively. A simulation test (or actual measurement) of theoptical scanning device of the present embodiment is performed, inadvance, and individual scanned-surface focal-point (or beam-pitch)deviations of the laser beam focused by the scanning optical unit aremeasured at different amounts of movement of the first corrector lens22. The gradients “a12”, “a22” and “a32” defining the respectiverelationships are created based on the results of the measurement of thesimulation test, and stored in the memory of the deviation calculatingunit 29.

Further, FIG. 15 shows a relationship between main-scanning-directionfocal-point deviation and second corrector lens movement in the presentembodiment. In FIG. 15, “a13” indicates a gradient of themain-scanning-direction focal-point deviation to the second correctorlens movement. FIG. 16 shows a relationship betweensub-scanning-direction focal-point deviation and second corrector lensmovement in the optical scanning device of the present embodiment. InFIG. 16, “a23” indicates a gradient of the sub-scanning-directionfocal-point deviation to the second corrector lens movement. FIG. 17shows a relationship between sub-scanning-direction beam-pitch deviationand second corrector lens movement in the optical scanning device of thepresent embodiment. In FIG. 13, “a33” indicates a gradient of thesub-scanning-direction beam-pitch deviation to the second corrector lensmovement.

In the present embodiment, the memory of the deviation calculating unit29 further stores the gradient “a13” defining the relationship betweenmain-scanning-direction focal-point deviation and second corrector lensmovement, the gradient “a23” defining the relationship betweensub-scanning-direction focal-point deviation and second corrector lensmovement, and the gradient “a33” defining the relationship betweensub-scanning-direction beam-pitch deviation and second correctormovement, respectively. A simulation test (or actual measurement) of theoptical scanning device of the present embodiment is performed, inadvance, and individual scanned-surface focal-point (or beam-pitch)deviations of the laser beam focused by the scanning optical unit aremeasured at different amounts of movement of the second corrector lens23. The gradients “a13”, “a23” and “a33” defining the respectiverelationships are created based on the results of the measurement of thesimulation test, and stored in the memory of the deviation calculatingunit 29.

In the present embodiment, the memory of the deviation calculating unit29 stores a first map defining a relationship between temperature changeand a corresponding main-scanning-direction focal-point deviation of thelaser beam, a second map defining a relationship between temperaturechange and a corresponding sub-scanning-direction focal-point deviationof the laser beam, and a third map defining a relationship betweentemperature change and a corresponding sub-scanning-direction beam-pitchdeviation of the light beam, respectively. A simulation test of theoptical scanning device of the present embodiment is performed, inadvance, and individual scanned-surface focal-point (or beam-pitch)deviations of the laser beam focused by the scanning optical unit aremeasured at different temperatures of the scanning optical unit. Thefirst through third maps defining the respective relationships arecreated based on the results of the measurement of the simulation test,and stored in the memory of the deviation calculating unit 29. Duringthe measurement of the simulation test, the focusing effect of thecorrector lenses 22 and 23 on the multiple light beams from the lightsource unit 21 is fixed to a reference level and not varied.

FIG. 18 shows a relationship between main-scanning-direction focal-pointdeviation of the optical scanning device of the present embodiment andtemperature change. FIG. 19 shows a relationship betweensub-scanning-direction focal-point deviation of the optical scanningdevice of the present embodiment and temperature change. FIG. 20 shows arelationship between sub-scanning-direction beam-pitch deviation of theoptical scanning device of the present embodiment and temperaturechange.

Suppose that “M” denotes a main-scanning-direction focal-point deviationof the light beam on the scanned surface for a temperature change, “S”denotes a sub-scanning-direction focal-point deviation of the light beamon the scanned surface for the temperature change, and “P” denotes asub-scanning-direction beam-pitch deviation of the light beam on thescanned surface for the temperature change. Further, suppose that “X1”denotes a first amount of movement of the first corrector lens 22 alongthe optical axis needed to cancel the deviation due to the temperaturechange, “X2” denotes a second amount of movement of the second correctorlens 23 along the optical axis needed to cancel the deviation due to thetemperature change, and “X3” denotes a third amount of movement of thelaser-diode unit 21 along the optical axis needed to cancel thedeviation due to the temperature change. Then, the following formula issatisfied. $\begin{pmatrix}M \\S \\P\end{pmatrix} = {\begin{pmatrix}a_{11} & a_{12} & a_{13} \\a_{21} & a_{22} & a_{23} \\a_{31} & a_{32} & a_{33}\end{pmatrix}\mspace{11mu}\begin{pmatrix}{X1} \\{X2} \\{X3}\end{pmatrix}}$From the above formula, the respective amounts “X1”, “X2” and “X3” ofthe corrector lens movement and the laser-diode unit movement that mustbe achieved in order to eliminate the focal-point and beam-pitchdeviations due to the temperature change, are calculated as follows.$\begin{pmatrix}{X1} \\{X2} \\{X3}\end{pmatrix} = {\begin{pmatrix}a_{11} & a_{12} & a_{13} \\a_{21} & a_{22} & a_{23} \\a_{31} & a_{32} & a_{33}\end{pmatrix}^{- 1}\;\begin{pmatrix}M \\S \\P\end{pmatrix}}$

In the present embodiment, by using the above formula, the deviationcalculating unit 29 calculates the first amount X1 of movement of thefirst corrector lens 22, the second amount X2 of movement of the secondcorrector lens 23, and the third amount X3 of movement of the lightsource unit 21, respectively, from the stored gradient matrix (a11through a33) and the respective deviations M, S and P of the first,second and third maps read from the memory in response to thetemperature change.

As described above, in the present embodiment, the simulation test isperformed, in advance, such that individual scanned-surface focal-pointor beam-pitch deviations of the laser beam focused by the scanningoptical unit are measured at different temperatures of the scanningoptical unit. The first through third maps defining the respectiverelationships are created based on the results of the measurement of thesimulation test, and the maps, such as shown in FIG. 18 through FIG. 20,are stored in the memory of the deviation calculating unit 29.

Accordingly, in the optical scanning device of FIG. 8, when atemperature change of the scanning optical unit is detected by thetemperature measuring unit 28, the beam-diameter control unit 30 adjustseach of the main-scanning-direction focal-point position and thesub-scanning-direction focal-point position of the light beam on thescanned surface by varying the focusing effect of the first correctorlens 22 on the light-source-emission light beam by the first amount X1of movement of the first corrector lens 22 along the optical axis, whichcorresponds to the temperature change and is supplied from the deviationcalculating unit 29, and varying the focusing effect of the secondcorrector leans 23 on the light-source-emission light beam by the secondamount X2 of movement of the second corrector lens 23 along the opticalaxis, which corresponds to the temperature change and is supplied fromthe deviation calculating unit 29. At the same time, the beam-pitchcontrol unit 31 adjusts the sub-scanning-direction beam pitch of thelight beam on the scanned surface by moving the light source unit 21along the optical axis by the third amount X3, which corresponds to thetemperature change and is supplied from the deviation calculating unit29.

It is not necessary for the optical scanning device of the presentembodiment to perform the automatic focusing operation when thetemperature of the scanning optical unit changes. In the presentembodiment, it is possible to provide one-to-one correspondence betweenthe temperature of the scanning optical unit and the correspondingamount of movement of each of the light source unit 21, the firstcorrector lens 22 and the second corrector lens 23.

The movement of each of the first corrector lens 22 and the secondcorrector lens 23 controlled by the beam-diameter control unit 30results in the elimination of the focal-point deviation corresponding tothat read from the memory of the deviation calculating unit 29 inresponse to the temperature change. The movement of the light sourceunit 21 controlled by the beam-pitch control unit 31 results in theelimination of the beam-pitch deviation corresponding to that read fromthe memory of the deviation calculating unit 29 in response to thetemperature change. In the optical scanning device of the presentembodiment, the temperature compensation unit can be constructed in asimple, inexpensive configuration. The optical scanning device and theimage forming apparatus of the present embodiment are effective inquickly achieving the optimum focal-point position and the optimum beampitch of the light beam on the scanned surface of the photosensitivemedium 6 when a temperature change of the scanning optical unit isdetected.

In the above-described embodiment, the temperature sensor 27 is providedin the vicinity of the photosensitive medium 26 as shown in FIG. 8. Thepresent invention is not limited to this embodiment. In order toincrease the accuracy of temperature detection, a plurality oftemperature sensors may be provided at different internal locationswithin the optical scanning device which are subjected to significanttemperature changes. In such embodiment, the temperature measuring unit28 detects the temperature of the scanning optical unit and itsneighboring locations by obtaining a weighted average of respectivetemperatures sensed by the plurality of temperature sensors. Theweighting factors of the temperature sensors for use in the calculationof the weighted average may be determined depending on the location ofeach temperature sensor. As the focusing lens system 25 and the lightsource unit 21 are significantly affected by temperature changes, it isnecessary to assign a relatively large weighting factor for thetemperature sensors if they are provided in the vicinity of the focusinglens system 25 or the light source unit 21.

The optical scanning device of the above-described embodiment isprovided in an image forming apparatus in which an image is formedthrough an electrophotographic printing process. In theelectrophotographic printing process, there are basically six majorsteps employed: (1) charging of the photosensitive medium; (2) exposingof the photosensitive medium to the image light pattern; (3) developingof the photosensitive medium with toner; (4) transferring of the tonedimage from the photosensitive medium to the final medium (usuallypaper); (5) thermal fusing of the toner to the paper; and (6) cleaningof residual toner from the photosensitive medium surface. The scanningof the photoconductive medium surface, performed by the light beam fromthe optical scanning device of the above-described embodiment,corresponds to the exposing step of the electrophotographic printingprocess that is carried out by the image forming apparatus.

The present invention is not limited to the above-described embodiment,and variations and modifications may be made without departing from thescope of the present invention.

Further, the present invention is based on Japanese priority applicationNo. 11-333510, filed on Nov. 24, 1999, and Japanese priority applicationNo. 2000-023930, filed on Feb. 1, 2000, the entire contents of which arehereby incorporated by reference.

1. An optical scanning device comprising: a light source unit having aplurality of light sources emitting multiple light beams; a scanningoptical unit deflecting the multiple light beams from the light sourcesand focusing the deflected light beams so that each light beam forms alight spot on a scanned surface, the scanned surface being scanned byeach light beam from the scanning optical unit; a temperature detectionunit detecting a temperature of the scanning optical unit and itsneighboring locations; and a temperature compensation unit adjustingeach of a main-scanning-direction focal-point position related to eachlight beam, a sub-scanning-direction focal-point position related toeach light beam, and a sub-scanning-direction beam pitch of the lightbeams on the scanned surface in accordance with a change in thetemperature detected by the temperature detection unit, wherein thetemperature compensation unit is configured to move the light sourceunit along its optical axis by an amount of movement corresponding tothe temperature change to adjust the sub-scanning-direction beam pitch,and configured to move a plurality of corrector lenses along theiroptical axes by an amount of movement corresponding to the temperaturechange to adjust the main-scanning-direction focal-point position andthe sub-scanning-direction focal-point position respectively, andwherein a beam pitch is defined by a spacing between two adjacentscanned lines formed by simultaneously scanning/deflecting two lightbeams from the light source unit.
 2. The optical scanning deviceaccording to claim 1, wherein the plurality of corrector lensescomprises a first corrector lens provided to adjust themain-scanning-direction focal-point position, and a second correctorlens provided to adjust the sub-scanning-direction focal-point position.3. The optical scanning device according to claim 1, wherein thetemperature compensation unit includes a memory that stores a first mapdefining a relationship between the temperature change and acorresponding main-scanning-direction focal-point deviation of the lightbeam, a second map defining a relationship between the temperaturechange and a corresponding sub-scanning-direction focal-point deviationof the light beam, and a third map defining a relationship between thetemperature change and a corresponding sub-scanning-direction beam-pitchdeviation of the light beam, the temperature compensation unit adjustingthe main-scanning-direction focal-point position, thesub-scanning-direction focal-point position and thesub-scanning-direction beam pitch based on the respective deviations ofthe first, second and third maps read from the memory in response to thetemperature change.
 4. The optical scanning device according to claim 7,wherein the temperature detection unit includes a plurality oftemperature sensors provided at different internal locations within theoptical scanning device, the temperature detection unit detecting thetemperature of the scanning optical unit and its neighboring locationsby obtaining a weighted average of respective temperatures sensed by theplurality of temperature sensors.
 5. An optical scanning methodcomprising the steps of: emitting multiple light beams from a pluralityof light sources contained in a light source unit; deflecting themultiple light beams from the light sources by a scanning optical unit;focusing the deflected light beams by the scanning optical unit so thateach light beam forms a light spot on a scanned surface, the scannedsurface being scanned by each light beam from the scanning optical unit;detecting a temperature of the scanning optical unit and its neighboringlocations; and adjusting each of a main-scanning-direction focal-pointposition related to each light beam, a sub-scanning-directionfocal-point position related to each light beam, and asub-scanning-direction beam pitch of the light beams on the scannedsurface, in accordance with a change in the temperature detected in thedetecting step, wherein the light source unit moves along its opticalaxis by an amount of movement corresponding to the temperature change toadjust the sub-scanning-direction beam pitch, and a plurality ofcorrector lenses moves along their optical axes by an amount of movementcorresponding to the temperature change to adjust themain-scanning-direction focal-point position and thesub-scanning-direction focal-point position respectively.
 6. An imageforming apparatus in which an optical scanning device is provided, theoptical scanning device comprising: a light source unit having aplurality of light sources emitting multiple light beams; a scanningoptical unit deflecting the multiple light beams from the light sourcesand focusing the deflected light beams so that each light beam forms alight spot on a scanned surface of a photosensitive medium, the scannedsurface being scanned by each light beam from the scanning optical unit;a temperature detection unit detecting a temperature of the scanningoptical unit and its neighboring locations; and a temperaturecompensation unit adjusting each of a main-scanning-directionfocal-point position related to each light beam, asub-scanning-direction focal-point position related to each light beam,and a sub-scanning-direction beam pitch of the light beams on thescanned surface in accordance with a change in the temperature detectedby the temperature detection unit, wherein the temperature compensationunit is configured to move the light source unit along its optical axisby an amount of movement corresponding to the temperature change toadjust the sub-scanning-direction beam pitch, and configured to move aplurality of corrector lenses along their optical axes by an amount ofmovement corresponding to the temperature change to adjust themain-scanning-direction focal-point position and thesub-scanning-direction focal-point position respectively.
 7. An opticalscanning device comprising: a light source unit having a plurality oflight sources emitting multiple light beams; a scanning optical unitdeflecting the multiple light beams from the light sources and focusingthe deflected light beams so that each light beam forms a light spot ona scanned surface, the scanned surface being scanned by each light beamfrom the scanning optical unit; a temperature detection unit detecting atemperature of the scanning optical unit and its neighboring locations;and a temperature compensation unit adjusting a sub-scanning-directionbeam pitch of the light beams on the scanned surface, wherein thetemperature compensation unit adjusts the sub-scanning-direction beampitch in response to a change in the temperature detected by thetemperature detection unit, and wherein the temperature compensationunit is configured to move the light source unit along its optical axisby an amount of movement corresponding to the temperature change toadjust the sub-scanning-direction beam pitch.
 8. An optical scanningdevice comprising: a light source unit having a plurality of lightsources emitting multiple light beams; a scanning optical unitdeflecting the multiple light beams from the light sources and focusingthe deflected light beams so that each light beam forms a light spot ona scanned surface, the scanned surface being scanned by each light beamfrom the scanning optical unit; a temperature detection unit detecting atemperature of the scanning optical unit and its neighboring locations;and a temperature compensation unit adjusting a sub-scanning-directionbeam pitch of the light beams on the scanned surface; and a memory thatstores a map defining a relationship between predetermined temperaturechanges and corresponding sub-scanning-direction beam-pitch deviationsof the light beam, wherein the temperature compensation unit adjusts thesub-scanning-direction beam pitch based on a correspondingsub-scanning-direction beam-pitch deviation of the map read from thememory in response to a change in the temperature detected by thetemperature detection unit, and wherein the temperature compensationunit is configured to move the light source unit along its optical axisby an amount of movement corresponding to the temperature change toadjust the sub-scanning-direction beam pitch.
 9. An optical scanningdevice comprising: a light source unit having a plurality of lightsources emitting multiple light beams; a scanning optical unitdeflecting the multiple light beams from the light sources and focusingthe deflected light beams so that each light beam forms a light spot ona scanned surface, the scanned surface being scanned by each light beamfrom the scanning optical unit; a temperature detection unit detecting atemperature of the scanning optical unit and its neighboring locations;and a temperature compensation unit adjusting a sub-scanning-directionbeam pitch of the light beams on the scanned surface, wherein thetemperature detection unit comprises a plurality of temperature sensorsprovided at different internal locations within the optical scanningdevice, the temperature detection unit detecting the temperature of thescanning optical unit and its neighboring locations by obtaining aweighted average of respective temperatures sensed by the plurality oftemperature sensors, wherein the temperature compensation unit adjuststhe sub-scanning-direction beam pitch in response to a change in thetemperature detected by the temperature detection unit, and wherein thetemperature compensation unit is configured to move the light sourceunit along its optical axis by an amount of movement corresponding tothe temperature change to adjust the sub-scanning-direction beam pitch.